power



Feb. 14. 1956 a. D. PowER 2,734,679

VAPOUR VACUUM PUMPS Filed Aug. 25, 1952 2 Sheets-Sheet l Feb. 14, 1956 B. D. POWER 2 Sheets-Sheet 2 lave/fr,- ddf/ (poll/017 *E1/verl United States Patent O VAPOUR VACUUM PUMPS Basil Dixon Power, West Wickham, England, assignor to Edwards High Vacuum Limited, Crawley, England Application August 25, 1952, Serial No. 306,160

Claims priority, application Great Britain August 29, 1951 6 Claims. (Cl. 230-101) This invention relates to vapour vacuum pumps and is more particularly, although not exclusively concerned with pumps of thecommon vapour vacuum type in which air to be exhausted is entrained in a stream of vapour of a suitable low vapour pressure fluid.

In` one form of pump of the kind referred to, the Vapour is supplied from a boiler, ascends a central tube and enters an annular jet at the top of the'tube from which it issues downwards into a space between the central tube and a cooled outer tube upon which it is condensed and returns to the boiler. Two or more such jets in series may be used. The upper end of the Outer tube constitutes the inlet to the pump and the outlet consists of a pipe connecting the lower part of the annular space between the central and outer tubes to a backing pump. Air exhaustion is eiected by the diffusion of the air molecules above the annular jet or jets, downwards into the stream of vapour molecules which are travelling in the same direction and at comparable speeds. Once entrained in the vapour stream, the air molecules cannot dituse back upwards in the face of the downward streaming vapour molecules, but are driven downwards and compressed towards the tube leading to the backing pump.

The lowest pressure obtainable by the use of pumps of the kind referred to is, to some extent, limited by the qualities of the available operating fluids.

In order to provide a pump capable of working against reasonably high backing pump pressures, it is necessary to boil the uid at a quite high temperature in the main boiler and to ensure adequate mechanical strength a metal construction is usual.

The high temperature and the catalytic action of the metal used, promote fluid decomposition so that gaseous and vaporous decomposition products are continually evolved. These products may issue from the vapour jets responsible for producing the lowest pressures and may impair the ability of the pump to produce low pressures. Similar eects may result if the pump iluid contains initial impurities or if gases dissolve in the tluid as it returns to the boiler through the region exposed to the backing pump pressure.

In multi-stage vapour pumps, various fractionating and purifying devices have been used to limit the effects of decomposition and impurity of the operating fluid and individual separately heated boilers have been used which permit the high vacuum stages to be fed from a boiler at low temperature while the stages in the backing pump region are fed from a boiler at higher temperature.

According to the present invention, in a vapour vacuum pump having at least two stages, there is provided, in addition to a main boiler, at least one auxiliary boiler, the liquid for which is provided solely by condensation of vapour from the main boiler, or in the case of a pump having two or more auxiliary boilers, from the preceding auxiliary boiler, the disposition of the, or each, auxiliary boiler being such that it is heated mainly or entirely by vapour from the main or a preceding aux- "ice iliary boiler. According to a feature of the invention the auxiliary boiler or boilers operate at a relatively low pressure and therefore at a relatively low temperature compared with the main boiler and serve to supply the high vacuum stages, while the stages at the backing pump end are fed from the higher temperature or main boiler.

The application of the invention to a diiusion pump enables it to be operated with a high backing pressure and although in the absence of the invention this would involve a high boiler pressure and temperature in the last stages of the pump, with consequential undesirable decomposition effects which might otherwise impair the ultimate vacuum obtainable from the high vacuum stages, the provision of the auxiliary boiler or boilers operating at decreased pressure vand temperature `and consequent less risk of decomposition, enables the desired high vacuum tobe obtained. Moreover, the least volatile constituents of a mixed operating luid condense preferentially to supply the auxiliary boiler, so that the high vacuum stages tend to be supplied from these most desirable components of the lluid.

According to a feature of the invention only one primary source of heat need be provided to supply' both or all of the boilers, although more than one heater may be provided, if desired.

According to a further feature of the invention the, or each of the auxiliary boilers may be composed of the least actively catalytic material available and which in other respects is suitable for its purpose. Glass may for example be used.

In order that the invention may be more clearly understood alternative forms of pump constructed in accordance therewith will now be described in greater detail with reference to Figs. 1 to 5 of the accompanying drawings of which Figs. l to 4 each lrepresent a pump diagrammatically and Fig. 5 is a cross-sectional view of a detail of an auxiliary boiler. y

Referring .to Fig. 1 of the drawings, the pump shown is provided with a main boiler 1 heated by a heater 2, a cooling water tube 3, a pipe 4 leading to a backing pump, not shown, and upper and lower conventional umbrella type jet stages 5 and 6 respectively. The condensed fluid from the jet stages returns to the boiler 1 where it is vaporised `in the well known manner. The boiler may run at a quite high temperature so that ther-l mal decomposition of the iluid may occur in the boiler. It is also recognised that high vapour pressure impurities and dissolved air may also enter the boiler 1 with the, returning lluid.

In applying the present invention, an auxiliaryboiler 7 is provided and, in the example shown, is disposed at the lower end of the vapour tube 8 leading to the jet 5. The vapour from boiler 1 passes the boiler 7 on its way to the main jet and the heavier vapour fractions condense on the outer wall of the vapour tube 8, run down into theA gutter formed between the lower end of the vapour tube and the turned up edge of the base ofthe boiler 7 and then into the interior of the boiler.

Any excess condensate overows from the gutter and returns to the main boiler 1.

The boiler 7 will be maintained so hot by the vapours arising from the main boiler 1 and bythe heavier vapour fractions which condense and ilow into it, that the light' fractions will tend not to condense on its outer wall andy flow into it but they will carry out their function of providing the vapour stream for the adjacent jet. The entry the main boiler 1 and supplies purified vapour comparatively free from decomposition products to the high vacuum jet 5. The temperature and pressure in the auxiliary boiler can be controlled by varying the outside area of the boiler available for condensation of the vapours which heat it and also by varying the throat area of the jet 5 which of course affects the boiler pressure and hence temperature.

It will be seen that the position occupied by the auxiliary boiler renders it Well protected from accidental breakage so that it can conveniently be made of glass or other comparatively noncatalytic material. its compara tively low temperature and its comparatively small uid capacity also help to reduce the rate of evolution of decomposition products from it.

Referring now to Fig. 2 of the drawings, a second arrangement is shown which is similar in principle to the arrangement shown in Fig. 1, but in the case of Fig. 2, the top half of the auxiliary boiler 9 is formed by part of the cone of the jet 6. The bottom half of boiler 9 is formed by an inverted cone 10 with a small hole 11 at its apex and the gutter illustrated in Fig. l is replaced by a small cup 12 supported under the apex of the inverted cone.

In operation, liquid is supported at a suitable level in boiler 9 because ofthe difference of pressure existing between boiler 9 and in the main boiler 1 below it.

In Fig. 3 of the drawings is shown a third possible arrangement of the auxiliary boiler. ln this example a splash bal'le 13 is provided comprising a disc-like member disposed below and' spaced from an annular member secured to the wall of the boiler 1. The purpose of this splash baffle is to prevent undesirable constituents of the iluid and its decomposition products, which would not normally condense on the auxiliary boiler, from reaching by means of direct splash from the main boiler.

In Fig. 3 the arrows show the direction taken by the vapour stream on its way to the main jet. The heaviest vapour fractions condense on the underside of the conical base 14 of an auxiliary boiler 15 and run down into a gutter 16' which is in the form of an annular trough. The boiler construction shown ensures that the streams of vapour and condensed uid' iiow in the same direction and any sweeping action of the vapour stream on the lm of condensed fluid tends to assist the flow of condensate into the gutter. By this means the uid lm, which has a low thermal conductivity, is kept as thin as possible so as to improve the heat transfer to the auxiliary boiler and to promote ample condensation.

Further condensation of vapour can also take place on the walls 17 of the auxiliary boiler, the condensate again running down into the gutter 16. Any excess condensate overflows from the gutter and returns to the main boiler.

Fig; 4 of the drawings shows a fourth arrangement of the auxiliary boiler, which is similar in principle to the arrangement shown in Figs. 1, 2 and 3. In the case of Fig. 4 the base 18 of the auxiliary boiler 19, and the gutter 20, are flat, and sidewalls are dispensed with. With such an arrangement the parts 18 and 20 may conveniently be made of one piece of material, with suitable passages provided for liquid to flow through into the boiler 19. The function of the parts 18 and 20 may, for example, be performed by aflat, circular, inverted dish shaped plate, shown in enlarged cross-section in Fig. 5. Adjacent the periphery of the recessed portion of the plate a ring of small holes 21 is provided. The vapour condensed onA thev undersideof the plate forms a thin film of liquid which is blown towards the periphery of the plate by the action of the vapour clinging to the plate by surface tension and the movement of the vapour itself. On reaching the holes 21 some of the liquid passes up into the boiler 19 because thev pressure under the boiler plate is greater than that above it; the top jet of the pump working at a lower pressure than the bottom jet. As in the previous arrangements described, for the same reason the liquid inside the auxiliary boiler is maintained at a suitable level by the difference in pressure between the main and auxiliary boilers.

Also in Fig. 4 is shown a further auxiliary vapour tube 2.2 the purpose of which is two fold.

Firstly, in a tall vapour stream pump there may be considerable heat loss by radiation from the main vapour tube (23 in Fig. 4) and therefore there may be considerable condensation on the inside wall of this vapour tube from the vapour ascending inside it. If the pump is operating with a mixed fluid, the most phlegrnatic constituents, that is to say those constituents which are the most desir able for the top jets, are likely to condense preferentially on this inside wall and run back to the main boiler-so that a considerable proportion of these phlegmatic constituents may never ascend as far as the auxiliary boiler and may have no chance of entering it.

Accordingly the auxiliary vapour tube 22 is so arranged that the vapour ascending inside it washes over the condensing surfaces 18 of the auxiliary boiler. Moreover, the vapour ascending outside this auxiliary vapour tube but inside the main vapour tube 23 tends to keep the auxiliary vapour tube hot and the main vapour tube itself serves as la radiation shield for the auxiliary vapour tube. Condensation on the inner surfaces of the auxiliary vapour tube is, therefore, reduced and the phlegrnatic constituents of the vapour ascending inside it are more likely, by its use, to reach the condensing surfaces 1S of the auxiliary boiler.

It is desirable to point out that the bulk of the vapour ascending inside the auxiliary vapour tube goes to supply the lower stage in Fig. 4. Only some phlegmatic constituents condense on and enter the auxiliary boiler to feed the top stage. ri`he diameter of the auxiliary vapour tube may be so great that most of the liquid surface of the main boiler is contained inside it and the bulk of the vapour produced is produced inside it. Thus, the vapour ascending inside the auxiliary vapour tube may contain all constituents of the mixed fluid, the auxiliary boiler being relied upon to extract, by selective condensation, desirable constituents for its operation.

Secondly, the upper end of the auxiliary vapour tube may be so designed (frusto-conical in the example illustrated in Fig. 4) and so spaced from the condensing surfaces of the auxiliary boiler, that a comparatively high velocity is developed in the vapour stream as it flows in the space between the said upper end of the auxiliary vapour tube and the said condensing surface. This vapour stream, on its way to the main jet stage, tends to sweep any condensed liquid on the surface 18 rapidly towards the gutter 29, thus improving the heat transfer into the auxiliary boiler by keeping the low thermal conductivity lm of fluid as thin as possible, as already described.

The bathe 13 shown in Fig. 4 prevents splashes of fluid reaching the auxiliary boiler direct from the main boiler, and acts in the same way as the baffle 13 shown in Fig. 3.

The vapour tube 22 may be used. in conjunction with any type of auxiliary boiler, such as those shown in Figs. l, 2 or 3. In some cases with pumps of a certain size, its use may be dispensed with entirely.

In the case of a pump having multiple jets, a series of auxiliary boilers similar to those described may be provided, each of such additional boilers being situated in the vapour stream of the boiler below it.

While itY maybe found that the heat imparted to the auxiliary boiler or boilers by the vapour from the boiler below is suiiicient for optimum results, it will be understood that additional local auxiliary boiler heating by electrical heating elements may be provided.

I claim:

l. A vapour vacuum pump comprising an upwardly extendingV housing, an inlet at the top of said housing, an outlet adjacent the lower end of the housing, a main boiler positioned at ther bottom of the housing for containing the operating liquid of the pump, a-.heating means for said boiler, vapour tubes extending successively Vupwardly within said housing for conducting the vapour resulting from the heated liquid, a plurality of vertically spaced downwardly directed jets fed With vapour by the said vapour tubes, cooling means on said housing providing a cooled surface for condensing the vapour issuing from the jets whereby the condensed vapour flows down the housing to the main boiler, an auxiliary boiler at the lower end of a vapour tube other than the lowermost tube, and positioned in the path of vapour passing to a lower jet to be heated thereby, and means for condensing the heavier fractions of the Vapour passing to said lower jet and for feeding the heavier vapour fractions thus condensed, to the auxiliary boiler.

2. A vacuum pump as claimed in claim l wherein the said condensing means comprises a portion of the vapour tube leading from the auxiliary boiler to the respective jet.

3. A vapour vacuum pump comprising an upwardly extending housing, an inlet at the top of the housing, an outlet adjacent the lower end of the housing, a main boiler positioned at the bottom of the housing for containing the operating liquid of the pump, heating means for said boiler, a first vapour tube extending upwardly within the housing from said boiler, a first downwardly directed jet positioned at the upper end of said first vapour tube and fed thereby with vapour from the heated liquid in the boiler and constituting a low vacuum stage of the pump, a second vapour tube extending upwardly within the housing from said first tube, a second downwardly directed jet positioned at the upper end of said second Vapour tube and constituting a high vacuum stage of the pump, an auxiliary boiler positioned at the lower end of said second tube for supplying vapour to said second jet, and in the path of the vapour passing from the main boiler to the first jet to be heated thereby, a lower end portion of said second tube positioned in the path of the vapour passing to the first jet for condensing the heavier vapour fractions and for feeding the condensate to the auxiliary boiler and cooling means surrounding the upper portion of the housing for condensing on the wall of the housing the vapour issuing from the first and second jets whereby the vapour thus condensed descends the wall of the housing to be returned to the main boiler.

4. A vapour vacuum pump comprising an upwardly extending housing, an inlet at the top of the housing, an outlet adjacent the lower end of the housing, a main boiler positioned at the bottom of the housing for containing the operating liquid of the pump, heating means for said boiler, a main vapour tube extending upwardly within the housing from the said main boiler, a first downwardly directed annular jet at the upper end of said main tube, a second vapour tube extending upwardly from said rst jet, a second downwardly directed annular jet at the top of said second vapour tube, an auxiliary boiler positioned at the lower end of said second tube and extending downwardly into the main vapour tube, a base for said auxiliary boiler, an aperture in the said base and a receptacle positioned beneath the said aperture and spaced therefrom whereby the heavier fractions of the vapour passing from the main boiler to the first jet are condensed upon the outer surface of the auxiliary boiler extending into the main vapour tube to flow into the receptacle and into the auxiliary boiler through the aperture in the base thereof, the said auxiliary boiler being heated by the vapour passing through the main vapour tube to the rst jet; and cooling means surrounding the housing to provide a cooled surface for condensing the vapour issuing from the jets whereby the vapour thus condensed flows down the housing to be returned to the main boiler.

5. A vapour vacuum pump as claimed in claim 4 wherein a baflie is disposed in the main vapour tube adjacent to and vertically spaced beneath the base of the auxiliary boiler for causing the vapour passing through the main tube to the first jet to pass over the base of the auxiliary boiler in a direction to assist the ow of condensed vapor thereon towards the said receptacle.

6. A vapour vacuum pump comprising an upwardly extending housing, an inlet in the upper end of said housing, an outlet adjacent the lower end of the housing, a main boiler positioned at the bottom of the housing and adapted to contain the operating liquid of the pump, heating means for said boiler, a first vapour tube extending upwardly within the housing from said main boiler, a iirst downwardly directed annular jet at the upper end of said first tube, a second vapour tube extending from said first jet upwardly within the housing, a second downwardly directed annual jet at the upper end of said second vapour tube, an auxiliary boiler positioned at the lower end of said second vapour tube, a base for said auxiliary boiler the said base extending across the top of the first vapour tube, an aperture through said base leading to the interior of the auxiliary boiler, and a gutter member positioned beneath the said aperture, an auxiliary vapour tube extending upwardly within the said first vapour tube and spaced therefrom, means defining an outlet from said auxiliary tube, the said outlet being spaced vertically beneath the base of the auxiliary boiler and efective to cause the vapour passing through said auxiliary vapour tube to the said irst jet to sweep across the base of the auxiliary boiler in a direction towards the said aperture whereby the vapour condensed on the said base is caused to ow between the base and the gutter member and through the aperture into the auxiliary boiler; and cooling means surrounding the housing to provide a cooled surface thereon for condensing the vapour issuing from said irst and second jets, the vapour thus condensed flowing down the housing to return to the main boiler.

No references cited. 

